The present invention relates generally to the addition of alloying ingredients to molten metal and more particularly to the addition of liquid alloying ingredients to molten steel undergoing continuous casting.
In the continuous casting of molten steel, a descending stream of molten steel is directed from an upper container, such as a ladle, to a lower container, such as a tundish, and from there into a continuous casting mold. It is desirable to add the alloying ingredients to the descending stream of molten steel because this facilitates the mixing of the alloying ingredients into the molten steel. Certain alloying ingredients, such as lead, bismuth, tellurium and selenium, typically added to steel to improve the machinability thereof, have relative low melting points compared to steel and are prone to excessive fuming when added to molten steel. One expedient which has been employed when adding such ingredients to molten steel comprises enclosing the descending stream of molten steel within a vertically disposed, tubular shroud having vertical, peripheral walls horizontally spaced from the descending stream to define an unfilled, annular space between the shroud and the descending stream. The alloying ingredient is then directed into the descending stream inside the shroud.
Typically, the alloying ingredient is in a solid, particulate form, such as shot particles. The form in which the alloying ingredient is added is important because the amount added must be amenable to precise metering and the size and shape of the additive must be such as to assure rapid dissolution and dispersion of the alloying ingredient. Hence, the usual form of addition is either shot particles of carefully controlled size or wire or strip of uniform diameter.
When the alloying ingredient is in the form of wire or strip, a mechanical propelling device is usually employed to feed the wire or strip into the molten steel bath. When the solid alloying ingredient is introduced into the descending stream of molten steel in the form of shot, the shot is usually mixed with a compressed inert gas, such as argon, which acts as a propellant or transporting or carrying medium for the shot.
When the alloying ingredient is added to the molten steel in solid form, the molten steel must be maintained at a temperature substantially higher than that normally required for casting without the alloying ingredient, in order to insure melting and dissolution of the alloying ingredient. Additional heat energy is required to offset the heat loss and temperature drop caused by the melting of a solid alloying ingredient.
It is desirable to continuously cast molten steel at a temperature as low as possible, and the need to employ a higher temperature in order to insure the dissolution and dispersion of the alloying ingredient is therefore disadvantageous.
The addition of alloying ingredient in the form of shot, to a descending stream of molten steel, inside a surrounding shroud, and with the shot mixed with a pressurized, inert gas carrying medium, is disclosed in Rellis, et al., U.S. Pat. No. 4,602,949 entitled "Method and Apparatus for Adding Solid Alloying Ingredients to Molten Metal Stream", and the disclosure thereof is incorporated herein by reference. When a compressed gas is employed in this manner the compressed gas expands within the shroud and has a cooling effect therein.
A problem which can arise when employing an arrangement of the type described in said Rellis, et al. patent is the build-up of a skull of steel on the interior of the shroud. This is caused by the cooling effect of the expanding inert gas on droplets of molten steel which originate in the descending stream and impinge against the interior peripheral wall of the shroud. The cooling effect of the expanding pressurized inert gas causes the droplets to solidify on the interior of the shroud resulting in the build-up of the aforementioned skull, which of course, is undesirable.